Quantitive Analysis

You are asked to prepare 500. mL of a 0.250 M acetate buffer at pH 4.90 using only pure acetic acid (MW=60.05 g/mol, pKa=4.76), 3.00 M NaOH, and water. Answer the following questions regarding the preparation of the buffer.
Question 1: 1. How many grams of acetic acid will you need to prepare the 500 mL buffer? Note that the given concentration of acetate refers to the concentration of all acetate species in solution.
My Solution: .25*.5=.125*60.05=7.5g.
Question 2:2. What volume of 3.00 M NaOH must you add to the acetic acid to achieve a buffer with a pH of 4.90 at a final volume of 500 mL? (Ignore activity coefficients.)
10^(4.9-4.76)=1.38=a/b
so a=(1.38*.125)b
.1725b+b=.125moles
1.1725b=.125moles
b=.107moles.
3M=.107mole so mL=35.7. However this is incorrect.. Any help would be amazing!

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1. HOw is it .1725b +b

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